1. Field of the Invention
The present invention generally relates to motion picture projection and photographic apparatus and, more particularly, to projection apparatus in which a film strip is moving continuously through the projector or photographic equipment.
2. Prior Art
The prior art teaches a number of devices for producing a stationary projected image either on a screen or on photograpic film. The discussion of the present invention will be directed toward an embodiment for a projector, but it is clear to those having skill in the art that the present invention is equally applicable for editing and photographic devices. Some prior art devices provided reflecting means comprising semi-circular mirrors which are mounted for rotation and are tilted independently of each other by suitable cam members. Other prior art devices employ a plurality of mirrors arranged around a cylindrical drum which is rotated so that each successive image is reflected by each successive mirror. Examples of the above types of continuous film motion projectors are found in U.S. Pat. Nos. 2,506,198, 2,843,006, 2,770,163 and 3,067,284.
Improvements on the above-identified devices are found in related types of continuous film feed projection systems. In an improved continuous film feed device, images of the film frame and of a control aperture are projected on said mirror, the images being adjacent one another. The image of the control apertures is reflected from the mirror through a small aperture in an opaque shield and onto a photocell mounted behind the shield. The film frame moves continuously through the projection gate, the image of the control aperture moves progressively to illuminate a greater portion of the photocell. The photocell increases current to a galvanometer which pivots the mirror to counteract the movement of the film and thereby stabilizes the image on the screen. A shutter periodically interrupts the light to the photocell to allow the mirror to return to its initial position where it locks on to a succeeding frame image. In this device, the mirror shaft is integral with the galvanometer shaft so that variation in signal applied to the galvanometer will result in the rotational movement of the shaft. In operation, the incremental movement of the sprocket hole images causes the galvanometer shaft to rotate the mirror and thereby seek to track the film frame. The inadequacies of using a sensitive galvanometer are clear. Since movement of the film must be continuous, any variation in the current output of the photocell will result in an amplified variation in the output of the galvanometer. Since the tracking operation of the mirror is directly related to the operation of the galvanometer, the over-sensitive relationship between the two will result in poor synchronization between the rotation of the mirror and the continuous film movement.
In another device disclosed by the prior art, a perforated film strip is continuously advanced through a curved film gate. The film is driven by a capstan and pressure roller imposing direct pressure upon the film. Light is transmitted through a lens system and an opening in the shutter. Projected light transmitted through the film is intercepted by an oscillating mirror which is fixed on a shaft. The reflecting surface of the mirror passes through the mirror's pivot axis at the center of curvature of the film gate. Light reflected from the mirror passes through an objective lens and is reflected by a stationary mirror towards a viewing screen. In this system, the oscillation of the mirror is synchornized with the output drive of the motor. An electromechanical clutch is energized by the detection of the presence of a film discontinuity such as a sprocket hole. The clutch causes the rotation of the shaft which in turn winds a spring. When the clutch is de-energized at the completion of a movement cycle of the oscillating mirror, the shaft upon which the mirror is mounted is immediately rotated by the spring in a direction opposite to that of the first rotation. As a result, the mirror is returned to its starting position for the next cycle. The use of a clutch and a return spring obviously interject unwanted problems into the system. The mechanical engagement and disengagement of the clutch as well as the snap back action of the mirror will interject excessive noise in a manner which is intolerable for film projection and editing. In addition, the direct pressure imposed upon the film can cause damage to the film and as a result be deleterious to the total operation of the system.
Another film motion compensator disclosed by the prior art utilizes a rotating mirror mechanism. In this system, light is to be reflected from a mirror which is pivotally supported about an axis which is coincident with its reflecting surface. Oscillating movement is imparted to the mirror by a lever which has a small hook pivoted to its end in the position for engagement with the sprocket perforations of the film while it passes over an arcuate guide. The film is moved at a constant rate of speed over the arcuate guide. A small spring urges the hook towards its position of engagement with a sprocket hole in the film so that movement of the film swings the lever and the mirror in a counterclockwise direction. This cycle is repeated as the film moves and causes the image, which is focused at the surface of the film, to move the film to create a highly resolved record of the film during its uninterrupted motion. The retrace of the mirror is caused by the spring when the lever is disengaged. This system utilizes no means to block the output of the light source since it is assumed that the retrace movement of the mirror is so rapid that the effect of light upon the film during this period is negligible. The inadequacies with this system are apparent. The mechanical movement of the lever and retrace spring will impose undue stress upon the film sprockets themselves as well as result in movement which cannot accurately track a film frame. In addition, the fly-back of the mirror utilizes the spring tension of the spring and thereby interjects noise components which the present invention seeks to obviate.
The present invention substantially resolves the difficulties which still remain in the continuous film motion projector disclosed by the prior art. The present invention utilizes a film movement mechanism which in no way bears upon the film frame and thereby protects the surface of the celluoid film The film is moved over a projection gate at a constant rate of speed, the same motive force used to move the film being employed to provide the motive force to the projecting mirror. To avoid the interjection of unwanted noise, a series of electromagnets or a slot cam are rotated in synchronism with the film movement mechanism. The oscillating mirror is aligned with the projection gate, the mirror rotating on a shaft which is in axial alignment with the electromagnet assembly. At the terminus of the shaft opposite that of the mirror, a ferromagnetic member is disposed in alignment with the poles of the electromagnet. Sequencing of the electromagnet will provide for rotation of the mirror in precise synchronism with the film movement, the retrace of the mirror being carried out without the use of any mechanical springs or like elements. The present invention will provide for accurate tracking of film frames, will be substantially noiseless and will provide for a stabilized film image for an interval of the total cycle which substantially surpasses the devices taught by the prior art.